The present invention relates to stabiliser tool and a wedge assembly for use in stabiliser tools that stabilise the operation of a drill string in a borehole drilling through the earth""s surface.
In operation boreholes are initially drilled as vertical holes and may then curve to an inclined or horizontal direction. Maintaining of complete control is very important but very difficult to achieve. A borehole drill may, for example, include three cutters or reamers, which are circumferentially spaced from one another. When the drill bit is inclined to the original axis, the space between the cutters can cause erratic or irregular operation in negotiating a curve.
So as to maintain control of the drill string, it is known to use a stabiliser tool having projecting wear pad assemblies. Generally a stabiliser tool includes recesses containing projecting pad assemblies arranged in a spaced configuration, with each pad assembly generally having two outer blocks and a central wedge block. The recesses have at least two side walls that diverge radially inwardly by an included angle providing an undercut recess surface. The outer blocks include diverging outer side walls of the same angle as the recess side walls. The outer walls are retained in abutment with the undercut recess surface by the wedge block acting on the inner walls of the outer blocks, the central wedge block having inwardly converging walls of the same angle as the diverging inner walls of the outer blocks. An appropriate bolt draws the central wedge into the gap between the outer blocks and causes them to move outwardly into firm abutment with the undercut recess thus locking them in place. The advantage of this configuration is that to repair or replace the wear pad assemblies one only needs to remove the central wedge block between the two outer blocks that then can be moved towards each other and away from the recess walls that locked them in place.
The difficulty with that configuration is that if the bolt locking the central wedge assembly in the recess were to break the central wedge may come away from the recess. This then allows the outer blocks to move away from the recess side walls and away from the recess. The end result is that the drill string may be jammed in a borehole, whereupon abandonment is unavoidable. In addition, not only is a multi-block wear pad assembly relatively expensive to manufacture, the manufacture of a recess with inwardly diverging side walls is also a complex task. Since the cost of the drill string is quite high it is an object of the present invention to provide improvements in stabiliser tools which overcome at least some of the abovementioned problems or provide a useful alternative.
In one aspect of the invention a drill string stabiliser tool has a body having an outer surface symmetrical about an axis of rotation, and a plurality of recesses extending inwardly from said outer surface;
said recesses each including opposite surfaces which converge inwardly from said body outer surface;
a plurality of pad assemblies in respective said recesses, each said pad assembly including a wedge block with converging radially inwardly opposite side surfaces complementary abutted by said recess surfaces;
each said wedge block having a relatively hard outer wear surface located radially outwardly from said body surface, and a bolt securing said wedge block to said body;
characterised by the outer side surfaces of the wedge block converging inwardly at a pre-determined angle and the opposite surfaces of said recesses converging inwardly at the same pre-determined angle whereby said angle is so chosen to provide an interference fit between the outer side surfaces of said wedge block and opposite surfaces of said recesses.
It has been surprisingly found that, even under immense forces that may be imposed on the wedge (wear) blocks and that are transmitted to the inwardly converging recess surfaces, the interference fit locks the wedge blocks in their recesses and resists the tendency of them to be removed by compressive forces, turning moments, or both even if the central bolt locking the wedge were to break. This then allows the drill string to be removed from the borehole and the pad assembly to be repaired without having to be abandoned.
This is an unexpected result for intuitively it is thought that a recess whose sides are inwardly converging would need a locking mechanism, such as a bolt to hold the wedge block in the recesses. It has previously been assumed that an engaging member such as a bolt passing through the assembly locks the pad assemblies in place. Accordingly there have been a number of proposals for secondary locking mechanisms to prevent the loss of a drill string. It has however been the unexpected discovery by the present inventor, that such an interference fit provides the necessary locking force to keep the assembly locked in place and that the central bolt is essentially only required to urge the wedge block inwardly and outwardly to achieve or break the interference fit.
To assist in achieving an interference fit, the total size (or width) of the wedge block is preferably slightly larger (wider) than that of the recess. This ensures that an interference fit is properly achieved when the wedge block is urged into the recess. Generally one would calculate the width of the recess and the wedge block would then be manufactured to be of slightly larger dimension than the dimensions of the recess. In preference it has been found that if the wedge block is {fraction (3/1000)} of an inch greater in width than the remaining width of the recess an effective interference fit occurs.
Thus to assemble the pad assembly a bolt is used to draw the central wedge into the recess resulting in an interference fit locking the assembly in place. To remove the pad assembly it is also necessary for the bolt to urge the wedge block outwardly and thus break the interference fit. Accordingly in use even if the central bolt were to break the interference fit has been found to lock the assembly in place.
In preference the predetermined angle depends on the material from which the assembly is manufactured. If made from steel, an angle of some 2 degrees has been found to produce an interference fit. However a range of angles of between 1.5 to 3.5 degrees has also been found to achieve an interference fit if the angle is greater or smaller than that range it has been found that an interference fit is not achieved and the wedge block is then only held in place by the bolt. If this were to break in a borehole than this could lead to the pad assembly becoming loose and being jammed in the borehole. Preferably the angle is some 2 degrees.
Further in the invention, there may be provided hard tipped cutters (for example polycrystallised diamond cutters) projecting outwardly from the sides of the wedge block, and if these are located towards the top end of the stabiliser and above the reamers of a drill string, the wall of a small bore hole will be enlarged upon withdrawal, thereby reducing likelihood of the drill string being jammed and lost in the hole. A negative rake of the cutters will still provide a cutting action, but may be effective in firmly compacting loose earth or rock upon withdrawal and avoid exacerbating the resistance to withdrawal by unnecessarily adding loose material between the cutters and reamers.
Preferably the wear blocks are positioned in a helical or spiral pattern that ensures that the drill stem upwardly of the reamers remains centralised, and during drilling, being above reamers (when used) the blocks will engage borehole surfaces which are more regular both in size and in smoothness than would exist if the reamers were not present. The stiffness below the stabiliser body of a reamer body and the bit assembly does much to avoid the difficulties previously encountered due to the spacing between the bits and the bit assembly, and thereby much improves the ability of an operator to control the direction of the bit movement.
In preference said wedge blocks are generally rectilinear in elevation, and side edges thereof are parallel in a plane normal to an axis of said bolt.
In preference each of some at least of said wedge blocks further include projecting hard tipped cutters.
In a further aspect of the invention there is proposed a drill string stabiliser tool including;
a) a body having an outer surface symmetrical about an axis of rotation, and a plurality of recesses extending inwardly from said outer surface and spaced in a helical configuration, said recess surfaces including opposite surfaces which converge inwardly from said body outer surface, each said recess being symmetrical about a central radial plane of said body;
b) a plurality of pad assemblies in respective said recesses, each said pad assembly having a wedge block having at least one pair of opposite sides converging towards said axis of rotation;
c) each said wedge block having a relatively hard outer wear surface located radially outwardly from said body surface, and a bolt securing said wedge block to said body;
e) each said wedge block containing surfaces defining a bolt hole extending from its said outer wear surface to a radially inner surface, slot surfaces extending transversely from a side of said wedge block defining a lateral slot extending across said bolt hole;
f) a U-shaped washer contained in said slot, and said bolt having a head bearing against said washer, accommodated in said slot, and accessible through said bolt hole from a said outer wear surface of said wedge block; wherein
the at least one pair of opposite sides of the wedge block converging inwardly at a pre-determined angle and the opposite surfaces of said recesses converging inwardly at the same pre-determined angle whereby said angle is so chosen to provide an interference fit between the outer side surfaces of said wedge block and opposite surfaces of said recesses.
In preference at least some of said wedge blocks include projecting hard tipped cutters, said hard tipped cutters being arranged in an array with the extent of projection of the cutters from a said block increasing from the trailing end of said block to the leading end thereof, with respect to a boring direction of drill string rotation.
In preference it is only the uppermost of said wedge blocks that have said projecting hard tipped cutters.
In a yet further form of the invention there is proposed a drill string stabiliser including;
a) an elongate body adapted for rotation about a body axis;
b) the body including an outer surface including a plurality of spaced perimetral recesses arranged in a helical array;
c) a plurality of wear pads each positioned in an associated one of the recesses;
d) each pad having spaced side surfaces each complemental with an associated side surface of the associated recess;
e) the side surfaces of each associated pad and recess converging oppositely from the body surface to a base of the recess at an obtuse angle whereby to provide retention of each such pad in its associated recess by an interference fit;
f) each pad including a wear surface for drill string guiding engagement with a wall of a bore hole; and
g) the complemental surfaces being position transversely of imaginary planes bisecting the body whereby forces applied to the mechanisms due to frictional engagement of the wear surfaces with a bore hole wall are applied transversely of the complemental surfaces.
In a still further from of the invention there is proposed a wear pad assembly for use with drill string stabiliser tools of the type having a plurality of recesses having inwardly converging side walls, said wear pad assembly including;
a wedge block having at least one pair of opposite side surfaces each side surface so shaped to complementary abut said recess surface;
each said wedge block having a relatively hard outer wear surface located radially outwardly from said body surface, and a bolt securing said wedge block to said body;
said outer side surfaces of the wedge block converging inwardly at a predetermined angle that is the same as the angle of the inwardly converging side walls of said recess, said angle being such so as to provide an interference fit between the outer side surfaces of said wedge block and the side walls of said recess.
Preferably said interference angle is between 1.5 to 3.5 degrees. Preferably the dimensions of the wedge block is slightly greater than that of the recess.